Vibrational adaptive sampling configuration interaction

Selected configuration interaction plus perturbation theory approaches have long been used to solve both the electronic and vibrational Schrödinger equations. In the last few years, many new selection algorithms have been developed for these approaches and applied to solve the electronic Schrödinger equation, but these algorithms have seen little to no use for solving the vibrational Schrödinger equation. Herein, we adapt one of the recently developed approaches, the adaptive sampling configuration interaction (ASCI) method, to calculate the vibrational excitations of molecules. The vibrational ASCI method has accuracy comparable to other high-accuracy approaches for solving the vibrational Schrödinger equation while requiring only modest computer resources. We study two different approaches for calculating excited states with vibrational ASCI and benchmark the method on acetonitrile and ethylene oxide. Finally, we demonstrate the applicability of the vibrational ASCI method to large systems by calculating the 128 lowest energy vibrational states of naphthalene, which has 48 vibrational degrees of freedom.